head restraint vs headrest

head restraint and headrest both are nouns.

Word	Noun	Adjective	Verb	Adverb
head restraint	Yes	No	No	No
headrest	Yes	No	No	No

As nouns, head restraint and headrest are synonyms defined as:

head restraint and headrest: a cushion attached to the top of the back of an automobile's seat to prevent whiplash

head restraint (noun)	headrest (noun)
a cushion attached to the top of the back of an automobile's seat to prevent whiplash	a rest for the head
	a cushion attached to the top of the back of an automobile's seat to prevent whiplash